Forensic DNA analysis of sexual assault evidence often involves analysis of DNA from sperm cells and DNA from other cells such as epithelial cells. Sperm cells are normally obtained from a rape victim by rubbing a swab against a mucous membrane. The samples obtained from victims often contain a mixture of sperm and epithelial cells. Because the epithelial cells may outnumber sperm cells in the sample by at least an order of magnitude, the former can cause contamination of sperm cell DNA, when sperm DNA is purified. Therefore, it is often desirable to separate, as cleanly as possible, the sperm cells and epithelial cells, or the sperm DNA and the epithelial DNA, prior to analysis. Separation and isolation of DNA from sperm and epithelial cells are essential steps in identifying an assailant from a forensic specimen, and in associating the assailant with the victim.
The standard method for purifying sperm from swabs is based on differential extraction. Separation of the sperm DNA from the victim's DNA removes ambiguity, facilitates DNA analysis and allows for easier interpretation of the assailant's DNA profile in a rape case. Although differential extraction is commonly used to separate sperm and epithelial cells, the standard protocol is time consuming and laborious.
Typically, cells are first resuspended from a forensic specimen, followed by selective digestion of the victim's epithelial cells with a solution containing Proteinase K and SDS (sodium dodecyl sulfate). The intact sperm are separated from the solubilized, contaminating DNA and epithelial cell debris by centrifugation, careful removal of supernatant, and extensive washing of the sperm pellet (see e.g., Giusti et al., J. Forensic Sci., 31:409-417, 1986; Gill et al. Nature 318:577-579, 1985; Wiegand et al., Int J. Legal Med., 104:359-360, 1992; and Yoshida et al., Forensic Sci. Int., 72:25-33, 1995). Unfortunately, the processes of centrifugation and careful removal of supernatant are difficult to automate and can cause the loss of sperm DNA due to multiple sample handling steps.
In one example of this procedure, Gill et al. (supra) describe a process for isolating sperm DNA from vaginal swabs taken from sexual assault victims. These swabs contain sperm and also a large excess of the victim's epithelial cells. The epithelial cells and the DNA contained in these cells is removed by preferential lysis (i.e., by incubation of the cell mixture in a buffer solution containing SDS, and proteinase K). Sperm nuclei are impervious to this treatment because they have disulfide bond cross-linked thiol-rich proteins, while other cell types are digested and the corresponding DNA is solubilized. After preferential lysis, the samples are centrifuged to separate the sperm nuclei from the victim's solubilized DNA. The supernatant containing the victim's DNA is removed and the sperm pellet is washed repeatedly. The sperm nuclei are subsequently lysed by treatment with a buffer solution containing SDS, proteinase K and DTT (dithiothreitol), and the lysate separated from the contaminating cells by centrifugation.
Wiegand et al. (supra) attempted to improve on the method of Gill et al. for samples having low sperm counts by using mild lysis conditions and by avoiding the washing steps.
A number of proposals for separating sperm cells from epithelial steps are based on filtration. Thus, Chen et al. (J Forensic Science 43:114-118, 1998) and Garvin (PCT/US01/01835) separate the sperm from the epithelial cells before differential lysis by gravitational or mild vacuum filtration or by use of a filter material that can withstand strong vacuum or centrifugal forces without having the pores increase in size. DNA is then isolated from the sperm collected in the filtrate.
A drawback of centrifugation-based methods is that centrifugation is difficult to automate. In addition, because of the loose nature of the cell pellets, the separation of supernatant containing DNA from the cell pellets is incomplete and causes cross-contamination between sperm cell and epithelial cell fractions. Centrifugation and vacuum steps can also damage the integrity of intact cells because of the harshness of the processes, thereby further reducing the likelihood of accurate results.
Attempts have also been made to use anti-sperm antibody coated magnetic beads (Eisenberg, A. J. “Development of a Spermatozoa Capture System for the Differential Extraction of Sexual Assault Evidence”; paper presented at: Profiling PCR and Beyond Conference, 2002; Washington, D.C.). Epitope stability, however, is a problem with this approach when applied to forensic work, because detergents such as Sarkosyl or SDS are required to efficiently elute sperm from the swabs and these detergents compromise epitopes recognized by the anti-sperm antibodies, reducing the number of sperm cells that can be separated and recovered.
In general, antibody-coated magnetic beads have been successfully used for many cell separation applications (Haukanes & Kvam, Biotechnology (NY). 11:60-63, 1993), but they have not been used to simply sequester cells in a process wherein DNA isolation is ultimately sought. Additional impediments may be present because human cells in forensic specimens have been dried onto an adsorbent substrate and then resuspended. In addition, because sperm lysis buffer can alter epitopes recognized by the anti-sperm antibodies, new antibodies recognizing different epitopes would most likely have to be employed, if sperm cell extraction is to be carried out with immuno-molecules.
Because of the difficulties of purifying sperm with immuno-molecules, size exclusion filters, such as the ones employed by Garvin and Chen (supra), have been employed.